ROUTING

Routing means finding a the route (next hop) for a datagram. If the destination node is on the same network as the source node, the delivery is direct. In direct delivery, the sender can compare the destination address with the address of the computers to which it is connected to. If a match is found, the packet is delivered to the same.
    If the destination host is not in the same network as the sender, the delivery is indirect. In the indirect delivery, the packet goes from router to router until it reaches the network of the destination host. In indirect delivery, the sender uses the IP address of the destination computer and a routing table. The roouting table is used to find the IP address of the router to which the packet should be sent to.

ROUTING METHOD'S

1. STATIC ROUTING

In static routing method, routing tables are manually configured by the network administrator. Static routing is used in smaller networks that contain only a smaller number of routers or where security is major concern. Routers that uses static routing are called static routers. Each static router must be configured and maintained separately because static routers do not exchange routing information with each other.

2. DYNAMIC ROUTING

Dynamic routing is a routing mechanism which is handled by routing protocols such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF) Protocol etc. These protocol dynamically exchange routing information among routers on an inter-network. Routers that use these methods are called dynamic routers.
    A router protocol is installed on each Dynamic Router. The routers periodically exchange their routing information so that if the inter-network is reconfigured or a router goes down, the routing tables of each router are modified accordingly.

Note -- Dynamic routers are less secure because routing tables can be hampered by         hackers

3. DISTRIBUTED ROUTING

In distributed routing, each router periodically exchanges its routing information with each of its neighbours. For example, let us assume that the hop count is taken as a metric-for-routing in a certain network. Once every T milli seconds, a router A send to its neighbourr a list of hop counts (to each destination). It also receives a similar list from its neighbours. Suppose a neighbours B has sent a routing table with 'Bi' as the hop count required to reach node i. If node A knows that the hop count to reach node B is m, it knows that it can reach node i via B in m + Bi hop counts. By performing similar calculation for all the routing tables that arrive, a node can find out the best estimate and correspondingly modify its routing table.

4. HIERARCHICAL ROUTING

In the method of routing, a network is divided into sub-networks. The router is a sub network knows only about the nodes within it sub-network. It is ignorant about the nodes in other sub-networks. This frees the routers from keeping information about all the nodes in a network. As networks grow in size, router does not require large memory, large CPU time and huge bandwidth.

5. DISTANCE VECTOR ROUTING (SHORTEST PATH ROUTING)

6. LINK-STATE ROUTING